ADS1120 DRDY pin not going low after Power-up

Dear Bob, I am using ADS1220 and I am facing the same issue as Richard - unreliable POR function. The recommended slow power supply ramp-up is difficult the achieve sometimes. In fact it requires an extra circuit to switch on the ADS1220 - quite annoying solution. Do you plan a silicon revision to fix the issue?

Hi Jan,

Welcome to the forum!  We do not plan to make any changes to the silicon.  In almost all cases this slower ramp is not an issue.  Unfortunately for you it is an issue, and one of the easiest ways to resolve is to add or increase the capacitance on DVDD which will slow the ramp rate as the cap is being charged.

I don't know why you would need an extra circuit to switch on the ADS1220, so perhaps there is another issue related to power that may be causing an issue.  I would be happy to review your schematic PCB layout to look for potential power related issues.

Best regards,

Bob B

Hi Bob, 

here goes partial schematics.

ADS1220 is powered from AVCC line which is directly tied to the P2.2 port of the MSP430.  The reason for switching ADS1220 on and off externally instead of using power-down and low-side switch feature is to minimize standby current.  Even in power-down the ADS1220 can draw several microamps which is quite much for me. 

My idea now is to use P2.1 and wire it via 500 ohm resitor to AVCC. In the software I would first put "1" on P2.1, wait 150 microseconds then put "1" on P2.2. This should result in slower ramp up. May be I can increase C1, too. Unfortunately this requires one more resistor and redesign of the PCB.

Unfortunately I have 100pcs already manufactured and they were intended to use for product certification. The requirement of the power supply slow ramp-up is quite new "feature" mentioned in the newest datasheet revision only. Unreliable POR behavior was hard to detect earlier in the desigh phase. All devices worked fine at room temperature. Later I noticed that one of ten pcs does not work at extereme temperatures (-20°C or +80°C) but I ascribed it to the poor quality of soldering which can be the case of the first prototypes manufactured in semiautomatic or even hand-operated process. 

I would like to ask you to provide as many details about the POR slow ramp-up "feature" as possible. For example: 1V/50us is fastest alowable ramp, right? Is there any limit on the other side, i.e. will POR operate correctly during very slow ramp-up? Where is the critical voltage region for the POR, i.e. is it possible to go up to 1V slower and then faster for example? Or is it critical all the way from zero to 2.3V ? Can I realize 1V/50us in small discrete steps?

One of my ideas is to use a kind of PWM on the P2.2 pin to slowly ramp-up the AVCC. Quite attractive idea at least as a temporary solution, because it can be done by software only and I could use already manufactured prototypes.  But this will cause the AVCC rise in small 0.1 or 0.2V steps. Although all the steps together do no exceed 1V/50us, locally each single step is faster than 1V/50us as shown on the picture below.  Will this be a problem? Please advice.

Thank you for understanding

Jan 

Hi Jan,

We did not see this issue during our characterization as the cap loading on our characterization board is much higher than on your board creating a slow enough ramp.  This problem was first discovered by another customer using a very similar design and requiring periods of complete power down to conserve power.  This customer also powered the ADS1220 and sensor from a microcontroller GPIO.  They too saw about a 10% power up failure.

In our analysis we found that some devices (due to process variation) did not complete the power up reset.  From over temperature testing and simulation we found that the 1V/50us would cover any issue with startup over the operating temperature range of the ADS1220.  We have not seen any issues with slower ramps as long as the ramp is monotonic.

Your operating case is one of those unusual operating conditions where the switching speed is so fast with the GPIO and board capacitance/inductance is low enough that the power up ramp is exceptionally fast.

Along with increased capacitance I did suggest a 2 stage start up using 2 GPIO with one creating a slow ramp using a series resistance and then completing the power up with a faster ramp with the second GPIO having no series resistance.  I have a little concern with the PWM start in that the supply might glitch or not be monotonic.  If filtered properly it may work.

The most critical period is point where the internal reset is issued which happens around 1.4 to 1.6V.  If you can obtain the slow ramp through this voltage most likely the device should power up correctly.  However the safest approach is 1V/50us ramp.

Another option would be to leave the power up the way it is and run a GPIO to the external clock pin.  If you issue approximately 32 clocks the ADS1220 is released from reset and will be able to communicate and you can send an SPI RESET command and go on from there.  The easiest thing is to just make sure your ramp is slow enough.

Best regards,

Bob B